In the above technical field, patent literature 1 discloses a technique of evaluating the continuity of a phase component in the time direction and smoothing an amplitude component for each frequency (paragraphs 0135 to 0138). Patent literature 2 describes detecting an abrupt frequency change by measuring a fluctuation in a phase in the time direction. Patent literature 3 describes, in paragraph 0024, that “a phase change in the complex vector of I and Q signals on a complex plane caused by superimposition of impulsive noise is always monitored, thereby reliably detecting the impulsive noise under a strong field environment”. The phase change is a change along the time axis. Patent literature 4 describes, in paragraph 0031, that “a phase linearizer 25 corrects a hop in a phase signal θ input from a polar coordinate converter 24 by linearization and outputs a resultant phase signal θ′ to a phase detector 26”. In addition, patent literature 4 has a description of a phase gradient detector in paragraph 0051, and also describes, in paragraph 0040, that “FIG. 5 shows an example of the input and output signals (a phase θ′ that is an input signal and a phase gradient dθ′ that is an output signal) of the phase detector 26”. Patent literature 5 discloses a technique of detecting an impulsive sound using an amplitude.